The invention relates to electron beam vaporisers.
Such electron beam vaporisers are known having an electron beam generator, a first magnetic deflection system with parallel, elongate pole plates for linear deflection of the beam along a first coordinate, and with a second magnetic deflection system for beam deflection along a second coordinate, which is perpendicular to the first coordinate, and also having an elongate vaporising crucible for holding the material to be vaporised.
Electron beams can be deflected practically without any inertia, so that the incident point or "spot" of the electron beam on a flat or curved surface can be precisely moved about. It is said that the electron beam can be moved in an X-Y-coordinate system. The picture tube of a television set is one of the best known examples of such a beam deflection.
It is however also known to utilise electron beams for technical production processes. Thus for example there is known from DE-AS No. 20 47 138 an electron beam vaporiser, which corresponds generally to the kind referred to above, but within any event a circular vaporising crucible. From the same publication it is however also known that as a result of the beam deflection, the focus and thus the energy density at the incident position of the electron beam can be altered in an undesired way. The known solution gives a compensation for this effect only by the dwell time, and not by a correction of the deflection pattern. Since alterations in focus at the incident position of the beam increase with increasing distance from the centre of the crucible, the known solution cannot be used to any effect for elongate vaporising crucibles that is to say for those such in which their length is at least 3 times greater than their breadth.
From DE-OS No. 28 12 285 is known an electron beam vaporiser of the above described kind in which by the cooperation of two magnetic deflection systems, electron beams can be guided in a particular linear pattern over the surface of an elongate vaporising crucible. In order in this way to heat the entire length of the vaporising crucible, two electron beam generators have to be provided, which each heat half of the vaporising crucible. The substrates here are situated stationary above the vaporising crucible, where they are continuously rotated. The scanning frequency of the surface of the bath in the vaporising crucible here plays only a relatively subordinate role since with a sufficiently long dwell time of the substrate in the vapour stream and with a plurality of rotations of the substrates, a satisfactory uniformity of deposition can be achieved.
A further problem is encountered however in the coating of bands or films of plastic, which nowadays are coated in a width of up to 1000 mm in one operation. In order to cover the entire length of the crucible with a single electron beam, a large deflection angle is needed. To avoid variations in the thickness of the layer in the lengthwise direction of the running band, the minimum deflection frequency of the electron beam is determined by the prescribed band velocity. With the known electron beam vaporisers, however one cannot achieve a satisfactorily uniform coating thickness distribution transverse to the running direction of the band, because the focus of the electron beam is unacceptably altered at a large deflection angle. In addition, the deflection pattern, in which the electron beam is guided over the surface of the material to be vaporised (the bath surface) is subject to marked distortions, which likewise have a deleterious influence on the uniformity of the distribution of coating thickness transverse to the running direction of the band.